This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-234767, filed Aug. 20, 1999; and No. 11-364609, filed Dec. 22, 1999, the entire contents of both of which are incorporated herein by reference.
The present invention relates to an optical fiber and an optical transmission line including the optical fiber, and more specifically an optical transmission line which can be used suitably in wavelength division multiplexing (WDM) optical communications.
As the optical transmission technique advances in terms of an increase in speed and capacity, the WDM transmission technique is attracting much attention as the mainstream technique. However, as the power of optical signal is enhanced, a new problem has started to occur, that is, a non-linear phenomenon which takes place due to the interaction between optical signals of two or more waves within an optical path.
Of the non-linear phenomenon, the four wave mixing (FWM) is considered to entail such a drawback that noise which occurs in the WDM transmission causes a serious adverse effect on the transmission, and therefore how to suppress it is being intensively studied. For example, OFC""94 Technical Digest PD19 proposes a dispersion shift optical fiber (DSF) which shifts the wavelength band to non-zero dispersion, as means for suppressing the non-linear phenomenon.
More specifically, such a DSF that has non-zero dispersion at a 1.55 xcexcm-wavelength band is used. In this case, the absolute value of the non-zero dispersion (unit: ps/nm/km) is, in many cases, set to 0.5 to 5.
Further, the distortion of a waveform caused by self phase modulation (SPM) and cross phase modulation (XPM) is another very serious problem. In the studies on how to solve such a problem, a research of suppressing the non-linear refraction index (n2) reported in OFC""97 TuNlb or the like, is studied, and further attention is paid to the technique for enlarging the mode field diameter (MFD) of the DSF, that is, the technique for enlarging the effective core area (Aeff) of the core.
The distortion xcfx86NL of a signal, which is caused by the non-linear phenomenon is expressed generally by the following formula (1):
xe2x80x83xcfx86NL=(2xcfx80xc3x97n2xc3x97Leffxc3x97P)/(xcexxc3x97Aeff)xe2x80x83xe2x80x83(1)
From the formula (1), it is understood that in order to decrease the distortion, xcfx86NL of a signal, which is caused by the non-linear phenomenon, Aeff should be large to be advantageous. Further, Aeff is expressed by the following formula (2):
Aeff=kxc3x97(MFD)2xe2x80x83xe2x80x83(2)
where k is a constant.
From the formula (2), when MFD is large, a low non-linearity can be obtained very efficiently.
As reported in OFC""96 WK15 and OFC""97 TuN2, the enlargement of MFD is presently one of the most required characteristics for the DSF.
Besides the non-linear phenomenon, the distortion of waveform due to dispersion is another problem in terms of the transmission characteristics of optical fiber. For the suppression of the distortion of the waveform due to the dispersion while suppressing the non-linear phenomenon, the method for managing the dispersion over the total optical line is effective. For example, in Jpn. Pat. Appln. KOKAI Publication No. 6-11620 proposes an optical transmission line achieved by combining a single-mode optical fiber (SMF) having zero dispersion at about 1.3 xcexcm and a dispersion compensation optical fiber (DCF).
Further, recently, an optical transmission line achieved by combining an SMF and a cable-type DFC is proposed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 10-325913.
In general, a DSF having zero dispersion or micro-dispersion at a 1.55 xcexcm-wavelength band has a high non-linearity and is easily influenced by XPM or SPM. As in the conventional case, a great number ofresearches have been made to reduce the non-linearity by enlargement of the MFD of the DSF; however the enlargement of the MFD of the DSF generally entails bending loss or an increase in dispersion slope. In the case of a DSF having non-zero dispersion at a 1.55 xcexcm-wavelength band, although its use at a wavelength band for zero dispersion is avoided, the wavelength dispersion per unit length (to be called local dispersion, hereinafter) is small, and therefore the FWM easily occurs as compared to the case of SMF.
On the other hand, the SMF has a larger positive local dispersion (about 16 ps/nm/km at a 1.55 xcexcm-wavelength band) than that of a DSF having non-zero dispersion at a 1.55 xcexcm-wavelength band, and therefore the FWM can be easily avoided. Further, since Aeff is relatively large (about 80 xcexcm2), a non-linear phenomenon such as XPM or SPM does not easily occur. Here, the deterioration of a signal waveform occurs due to large dispersion at a 1.55 xcexcm-wavelength band; however it can be solved by managing the total line with use of a dispersion compensation optical fiber such as described above. Further, in general, an SMF has a low loss and low PMD. That is, it can be said that an SMF is a relatively suitable fiber for the WDM transmission.
However, as the speed and capacity of data transmission is further increased in the future, very high power is input to a fiber and therefore even a present SMF might have a problem of non-linear phenomenon. Further, a dispersion compensation optical fiber for compensating dispersion of SMF has a high non-linearity due to its structure, and therefore a non-linear phenomenon such as XPM or SPM easily occurs.
Under these circumstances, an object of the present invention is to provide a positive dispersion optical fiber of a new type, which solves the above-described problems.
Another problem of the present invention is to provide an optical transmission line which includes such a positive dispersion optical fiber in its part.
According to the present invention, there is provided an optical fiber which has a dispersion value at a 1.55 xcexcm-wavelength band, of 6 to 24 ps/nm/km, and satisfies A greater than 3xc3x97D+40, where D represents a dispersion value (ps/nm/km) at a central wavelength of a 1.55 xcexcm-wavelength band, and A represents an effective core area (xcexcm2).
Further, according to the present invention, there is provided an optical transmission line for transmitting an optical signal, which includes an optical fiber, wherein at least a part of the optical fiber has a dispersion value at a 1.55 xcexcm-wavelength band, of 6 to 24 ps/nm/km, and satisfies A greater than 3xc3x97D+40, where D represents a dispersion value (ps/nm/km) at a central wavelength of a 1.55 xcexcm-wavelength band, and A represents an effective core area (xcexcm2).
The optical fiber of the present invention having the above-described structure has the following types.
(1) An optical fiber having: a dispersion value at a 1.55 xcexcm-wavelength band, of 17 to 24 ps/nm/km; an effective core area at a central wavelength of a 1.55 xcexcm-wavelength band, of 95 xcexcm2 or more, and a bending loss at a bending diameter of 20 mm, of 20 dB/m or less, and operating in a single mode at a 1.55 xcexcm-wavelength band.
(2) An optical fiber having: a dispersion value at a 1.55 xcexcm-wavelength band, of 14 to 17 ps/nm/km; an effective core area at a central wavelength of a 1.55 xcexcm-wavelength band, of 90 xcexcm2 or more, and a bending loss at a bending diameter of 20 mm, of 20 dB/m or less, and operating in a single mode at a 1.55 xcexcm-wavelength band.
(3) An optical fiber having: a dispersion value at a 1.55 xcexcm-wavelength band, of 6 to 14 ps/nm/km; an effective core area at a central wavelength of a 1.55 xcexcm-wavelength band, of 75 xcexcm2 or more, and a bending loss at a bending diameter of 20 mm, of 20 dB/m or less, and operating in a single mode at a 1.55 xcexcm-wavelength band.
(4) An optical fiber having a dispersion slope (unit: ps/nm2/km) at a 1.55 xcexcm-wavelength band, of 0.08 or less in absolute value.
(5) An optical fiber having a transmission loss at a central wavelength of a 1.55 xcexcm-wavelength band, of 0.25 dB/km or less, and a polarization mode dispersion value of 0.15 ps/km1/2 or less.
(6) An optical fiber having a transmission loss at an entire wavelength band of 1.55 xcexcm, of 0.25 dB/km or less.
(7) An optical fiber comprising a single layer core and clad, and having a refractive index profile of a single peaked structure, which satisfies 0.2%xe2x89xa6xcex941xe2x89xa60.35% where xcex941 is a relative refractive index difference of the core with reference to the refractive index of the clad.
(8) An optical fiber comprising a single layer core and clad, and having a refractive index profile of a single peaked structure, which satisfies 0.2%xe2x89xa6xcex941xe2x89xa60.6% where xcex941 is a relative refractive index difference of the core, with reference to the refractive index of the clad, and having an xcex1 value which satisfies 1xe2x89xa6xcex1xe2x89xa66 where xcex1 is a value obtained when the refractive index profile is approximated with an xcex1 curve.
(9) An optical fiber comprising a center core, a side core and a clad in order from an inner side, and having a refractive index profile of a two-layer core type, which satisfies 0.2%xe2x89xa6xcex941xe2x89xa60.35% and xe2x88x920.3%xe2x89xa6xcex942xe2x89xa60 where xcex941 is a relative refractive index difference of the center core, with reference to the refractive index of the clad, and where xcex942 is a relative refractive index difference of the side core, with reference to the refractive index of the clad, and satisfies 0.3xe2x89xa6a/bxe2x89xa60.7 where a represents an outer diameter of the center core and b represents an outer diameter of the side core.
(10) An optical fiber comprising a center core, a side core and a clad in order from an inner side, and having a refractive index profile of a two-layer core type, which satisfies 0.2%xe2x89xa6xcex941xe2x89xa60.7% and xe2x88x920.3%xe2x89xa6xcex942 less than xe2x88x920.1% where xcex941 is a relative refractive index difference of the center core, with reference to the refractive index of the clad, and where xcex942 is a relative refractive index difference of the side core, with reference to the refractive index of the clad, and satisfies 0.3xe2x89xa6a/bxe2x89xa60.7 where a represents an outer diameter of the center core and b represents an outer diameter of the side core, and has an xcex1 value which satisfies 1xe2x89xa6xcex1xe2x89xa66 where xcex1 is a value obtained when the refractive index profile is approximated with an xcex1 curve.
(11) An optical fiber comprising a center core, a side core and a clad in order from an inner side, and having a refractive index profile of a two-layer core type, which satisfies 0.2%xe2x89xa6xcex941xe2x89xa60.35% and 0 less than xcex942 less than xcex941 where xcex941 is a relative refractive index difference of the center core, with reference to the refractive index of the clad, and where xcex942 is a relative refractive index difference of the side core, with reference to the refractive index of the clad, and satisfies 0.3xe2x89xa6a/bxe2x89xa60.7 where a represents an outer diameter of the center core and b represents an outer diameter of the side core.
(12) An optical fiber comprising a center core, a side core and a clad in order from an inner side, and having a refractive index profile of a two-layer core type, which satisfies 0.2%xe2x89xa6xcex941xe2x89xa60.7%, 0.1%xe2x89xa6xcex942xe2x89xa60.3% and xcex941 greater than xcex942 where xcex941 is a relative refractive index difference of the center core, with reference to the refractive index of the clad, and where xcex942 is a relative refractive index difference of the side core, with reference to the refractive index of the clad, and satisfies 0.3xe2x89xa6a/bxe2x89xa60.7 where a represents an outer diameter of the center core and b represents an outer diameter of the side core, and has an xcex1 value which satisfies 1xe2x89xa6xcex1xe2x89xa66 where xcex1 is a value obtained when the refractive index profile is approximated with an xcex1 curve.
(13) An optical fiber according to (11), (12), wherein at least a part of the side core has a refractive index variation portion.
(14) An optical fiber comprising a center core, a side core and a clad in order from an inner side, and having a refractive index profile of a two-layer core type, which satisfies 0.6%xe2x89xa6xcex942xe2x89xa61.0% and xe2x88x921.2xe2x89xa6xcex941/xcex942xe2x89xa6xe2x88x920.4 where xcex941 is a relative refractive index difference of the center core, with reference to the refractive index of the clad, and where xcex942 is a relative refractive index difference of the side core, with reference to the refractive index of the clad, and satisfies 0.3xe2x89xa6a/bxe2x89xa60.7 where a represents an outer diameter of the center core and b represents an outer diameter of the side core.
(15) An optical fiber comprising a center core, a first side core, a second side core and a clad in order from an inner side, and having a refractive index profile of a three-layer core type, which satisfies 0.6%xe2x89xa6xcex942xe2x89xa61.0%, xe2x88x921.2xe2x89xa6xcex941/xcex942xe2x89xa6xe2x88x920.4 and 0.2xe2x89xa6xcex942/xcex943xe2x89xa60.6 where xcex941 is a relative refractive index difference of the center core, with reference to the refractive index of the clad, and where xcex942 is a relative refractive index difference of the first side core, with reference to the refractive index of the clad, xcex943 is a relative refractive index difference of the second side core, with reference to the refractive index of the clad, and satisfies 0.3xe2x89xa6a/bxe2x89xa60.7 and 0.2xe2x89xa6a/cxe2x89xa60.5 where a represents an outer diameter of the center core, b represents an outer diameter of the first side core, and c represents an outer diameter of the second side core.
(16) An optical fiber according to (15), wherein at least a part of the second side core has a refractive index variation portion.
It should be noted here that in the present specification, unless it is specifically indicated, the 1.5 xcexcm-wavelength band is meant to be a wavelength range of 1520 to 1620 nm, and the 1.55 xcexcm-wavelength band is meant to be a wavelength band of the 1.5 xcexcm-wavelength band, where signal is actually transmitted in an optical transmission line, which is, for example, a wavelength range of 1530 to 1570. Further, the band of the 1.55 xcexcm-wavelength band is supposed to be a band of 30 nm or higher, in consideration of being used in the WDM transmission.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.